1. Field of Invention
This invention relates to optical systems, and more particularly to improved spatial resolution along the optical axis of a coaxial optical system, primarily adapted for use in light scattering diagnostics.
2. Description of the Prior Art
The phenomenon known as Raman scattering has for many years been utilized as a laboratory tool in studying the structure of molecules and the characteristics of the solid state. With the recent increase of laser technology, highly sensitive photodetector systems, and processing capabilities, Raman scattering techniques are increasingly being applied to fluid mechanics and combustion and in such diverse arts as meteorology, atmospheric pollution monitoring, exhaust gas analysis, industrial process control, and the like. In a typical apparatus which will serve as an exemplary basis for discussion herein, Raman scattering techniques may be employed using a system which irradiates an object, such as a volume of hot gases in the exhaust effluent of a jet engine, with a finely collimated beam of energy, such as provided by a laser, while viewing a portion of the axis so illuminated with an optical system at right angles to the axis of the source. The optical system typically includes a collection lens or mirror (which may be comprised of a pair of individual lenses for improved control over aberrations, or may be a Cassegrainian primary), focusing lenses, optically responsive means to convert to electric signals, and some form of processing, in dependence on the particular phenomenon which is under study. A wide variety of systems of this general type, and the underlying theories of physics relating thereto, and adequately disclosed in Lapp and Penney, "Laser Raman Gas Diagnostics", Plenum Press: New York, 1974. Therefore, further detail relating to the known art of Raman gas diagnostics is not given herein.
Substantially all Raman experiments reported to date are performed with viewing geometries having very large angles (that is, that the viewing optics are at right angles or large angles with respect to the axis of the radiation source), for several reasons. First, Raman scattering is extremely weak, requiring ten orders of magnitude or more greater irradiation power than the power of the resulting optical signal to be sensed. As the angle decreases, and particularly where coaxial arrangements are used (in which the radiating source irradiates the specimen or sample volume from the same direction as the collecting optics views the specimen), the amount of spurious backscattered light increases dramatically. The weak Raman scattering effect is frequently completely swamped and unable to be detected amidst the high amount of backscattering in a coaxial configuration. A second problem with small angle viewing geometries is that the spatial resolution obtained by coincidence of two axes degrades markedly as the viewing angle approaches the irradiation angle. When the angles are the same (in a coaxial configuration), the inherent area along the axis of irradiation which is collected by the optic system (when a refractive, two-lens system is used) is from half the focal length to infinity. Since temperature profiles and other phenomena are liable to vary significantly over a small fraction of this collection area, there is a tendency for averaging or masking of the results being sensed to the point that no significant information can be derived by coaxial Raman scattering techniques. This effect is lessened somewhat in that the fractional contribution of light collected by an optical system is concentrated near the focal point; but since the effects being sensed can vary by two or more orders of magnitude at points quite close to the focal point, the actual responses in the gases being analyzed may in fact be far greater than the collection concentration ratio, such that the effect actually being monitored may be outside of the high-percentage region of the illuminating subject.
However, there are many instances where the utilization of large-angle (or right-angle) viewing geometries are undesirable. For instance, where only a single aperture is provided in an existing system, it may be totally impossible to provide a second aperture to permit a wide angle geometry system to be utilized for diagnosing a specimen within the aperture. Additionally, the specimen may be contained within an annular chamber or the like where the geometrical association of a wide-angle viewing geometry would be impossible.